OBD-II trouble code
P0312: Cylinder 12 Misfire Detected
Combustion failed repeatedly on cylinder 12. Unlike an injector or coil circuit code, this one reports a result rather than an electrical state — which widens the list of possible causes rather than narrowing it.
Quick facts
- System
- Powertrain
- Category
- Ignition / Misfire
- Severity
- High severity
- Drivable
- Usually safe to drive short-term
- Repair cost range
- $150 – $3,500
- DIY difficulty
- Advanced DIY
What does P0312 mean?
It is worth being precise about what separates this code from the circuit codes that share its cylinder number. P0212 reports that the injector driver circuit for cylinder 12 measured a voltage it should not have. P0362 reports the same about the coil driver. Those are electrical statements, and they are silent about whether the cylinder actually fired. P0312 makes the opposite kind of statement: combustion did not happen enough times to matter, and the module is not claiming to know why.
That difference sets the whole approach. A circuit code hands you a wire to test. A misfire code hands you a result and leaves three independent systems in play — spark, fuel and compression — any one of which can produce it with every circuit in the engine reading perfectly. A cylinder with 60 psi of compression, a plug bridged with oil, an injector that atomises into a stream instead of a mist, or a valve held fractionally open by a carbon deposit will all set P0312 without a single electrical fault code appearing anywhere.
The practical consequence is that guessing is unusually expensive here, and the cure is to run tests that eliminate whole systems rather than tests that check individual parts. Relative compression, read from the scan tool's own crankshaft data during cranking, eliminates or implicates the mechanical leg in a couple of minutes without removing anything. An injector balance or return-volume test does the same for fuelling. Only after two of the three legs are cleared does spending money on ignition components make sense.
One more habit is worth building on this code specifically: capture the freeze frame before clearing. Misfire is conditional. It appears at a particular combination of engine speed, load and temperature and disappears outside it, so the record of what the engine was doing when the code set is a set of instructions for reproducing the fault. Clearing codes first and going for a drive afterwards is the single most common reason a misfire is chased for weeks.
Common causes
- Ignition coil on cylinder 12 producing weak or no secondary output
- Fuel injector delivering the wrong quantity or a poor spray pattern rather than failing outright
- Spark plug fouled by oil or fuel, or eroded past its service gap
- Low compression from a burnt valve, a broken ring or a leaking head gasket
- Carbon deposits holding an intake valve fractionally off its seat
- Vacuum leak at the runner or gasket feeding that cylinder
- Damaged spark plug boot allowing spark to track to ground outside the cylinder
- Failed coil ground or corroded connector terminal on that branch of the harness
Symptoms
- Flashing check engine light while the misfire is occurring
- Rough idle that smooths out as engine speed rises, or the reverse
- Hesitation or a flat spot at a specific throttle opening
- Fuel economy noticeably worse than usual
- Smell of unburnt fuel at the tailpipe
- Slow or failed cranking of the emissions readiness monitors
- Elevated hydrocarbon reading if the vehicle is tailpipe tested
Diagnostic steps
- 1.Save and read the freeze frame first. Engine speed, calculated load, coolant temperature and short-term fuel trim at the moment of the set define the test drive you need to perform later.
- 2.Run relative compression from the scan tool using crankshaft speed during cranking. This is free, takes minutes, and either eliminates the mechanical leg or moves it to the front of the queue.
- 3.Perform an injector balance test or compare injector return volumes where the fuel system allows it. A partially blocked injector flows enough to look alive on a resistance check and still lean out its cylinder.
- 4.Check secondary ignition on that cylinder under load rather than at idle. Coils that measure fine cold and fine at idle routinely break down at high cylinder pressure, which is precisely the condition freeze frame told you to reproduce.
- 5.Inspect the spark plug when it comes out and read it rather than binning it. Oil fouling, coolant deposits and fuel washing each point at a different repair.
- 6.If ignition and fuel are clear, perform a cylinder leak-down test to distinguish valves from rings from a head gasket before any disassembly is authorised.
- 7.Confirm no crank or cam position code is stored. A misfire attributed to the wrong cylinder wastes every test above on a healthy cylinder.
Repair cost
$150 – $3,500
The spread is driven by which of the three systems is at fault. Ignition is the cheap end: a coil and plug fitted is $150 to $450. Fuelling is the middle: injector replacement is $400 to $1,000 on most applications and considerably more where the intake has to come off. Mechanical is the expensive end and there is no cheap version of it — valve work, ring wear or a head gasket on a twelve-cylinder engine starts around $3,000 and climbs. Roughly $120 to $250 of diagnostic time spent eliminating two of the three legs is the best value in the whole repair.
Estimate your repair
Run the numbers for your vehicle
Open the Repair Cost Estimator with ignition coil replacement preselected. Adjust labor rate and vehicle category to fit your situation.
DIY vs shop
This is an advanced DIY job. It typically requires specialty tools, scan-tool access, lifting equipment, or careful sequencing to avoid causing new failures. Plan for extended downtime and have a backup vehicle. Most owners are better served by a shop that has done this repair before.